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Most modern Fortran compilers allow two ways of writing continuation
lines. If the first non-space character on a line is in column 5, then
that line is a continuation of the previous line. We call this "fixed
format". (In GNU Emacs we always count columns from 0.) A line that
starts with a tab character followed by any digit except `0' is also a
continuation line. We call this style of continuation "tab format".
Fortran mode can make either style of continuation line, but you
must specify which one you prefer. The value of the variable
`indent-tabs-mode' controls the choice: `nil' for fixed format, and
non-`nil' for tab format. You can tell which style is presently in
effect by the presence or absence of the string `Tab' in the mode line.
If the text on a line starts with the conventional Fortran
continuation marker `$', or if it begins with any non-whitespace
character in column 5, Fortran mode treats it as a continuation line.
When you indent a continuation line with TAB, it converts the line to
the current continuation style. When you split a Fortran statement
with `M-LFD', the continuation marker on the newline is created
according to the continuation style.
The setting of continuation style affects several other aspects of
editing in Fortran mode. In fixed format mode, the minimum column
number for the body of a statement is 6. Lines inside of Fortran
blocks that are indented to larger column numbers always use only the
space character for whitespace. In tab format mode, the minimum column
number for the statement body is 8, and the whitespace before column 8
must always consist of one tab character.
When you enter Fortran mode for an existing file, it tries to deduce
the proper continuation style automatically from the file contents.
The first line that begins with either a tab character or six spaces
determines the choice. The variable `fortran-analyze-depth' specifies
how many lines to consider (at the beginning of the file); if none of
those lines indicates a style, then the variable
`fortran-tab-mode-default' specifies the style. If it is `nil', that
specifies fixed format, and non-`nil' specifies tab format.
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